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Early parenting key role in infants' physiological response to stress

September 17, 2008

Early parenting key role in infants' physiological response to stress

University Park, Pa. — In infancy, genes are the key influence on a child's ability to deal with stress, but as early as 6 months of age, parenting plays an important role in changing the impact of genes that may put infants at risk for responding poorly to stress, says a new study by researchers at University of North Carolina-Chapel Hill, Penn State, University of North Carolina-Greensboro and North Carolina State University.

In the September/October 2008 issue of the journal Child Development, the researchers published their findings of 142 infants who had been placed in a stressful situation — being separated from their mothers — when they were 3, 6 and 12 months old. They measured infants' heart rates while they were exposed to the stressor, isolating a cardiac response called vagal tone. Vagal tone acts like a brake on the heart when the body is in a calm state, but during a challenging situation, this brake is withdrawn, allowing heart rate to increase so the body can actively deal with the challenge.

They also collected DNA to determine which form of a dopamine receptor gene the infants carried; specific forms of this gene are related to problems in adolescence and adulthood including aggression, substance abuse and other risky behaviors. To assess the mothers' behavior as high or low in sensitivity, they also videotaped the mothers and their infants playing together for 10 minutes when the babies were 6 months old.

Both genes and parenting were found to be important to the infants' development of the way in which the brain helps regulate cardiac responses to stress. At 3 and 6 months old, those infants who had the form of the dopamine gene that has been associated with later risky behaviors did not display an effective cardiac response to the stressor (a decrease in vagal tone that takes the brake off the heart so that it can respond appropriately), while those infants with the non-risk version of the gene did. At these early ages, the researchers found, it didn't appear to matter whether mothers were sensitive or not.

However, by the time the infants were 12 months old, the pattern changed. Infants with the risk form of the gene who also had mothers who were highly sensitive now showed the expected cardiac response while they were exposed to the stressful situation. Those infants with the risk form of the gene who had insensitive mothers continued to show the ineffective cardiac response to the stressor. These findings suggest that although genes play a role in the development of physiological responses to stress, environmental experience (such as mothers' sensitive care-giving behavior) can have a strong influence, enough to change the effect that genes have on physiology very early in life. The researchers suggest this may be because of the cumulative effect on infants of exposure to their mothers' behavior.

"Our findings provide further support for the notion that the development of complex behavioral and physiological responses is not the result of nature or nurture, but rather a combination of the two," said Cathi Propper, research scientist at the University of North Carolina-Chapel Hill and the study's lead author. "They also illustrate the importance of parenting not just for the development of children's behavior, but for the underlying physiological mechanisms that support this behavior."

Ginger Moore, a study co-author and assistant professor of psychology at Penn State, added, "Lastly, infancy is an important time for developing behavioral and biological processes. Although these processes will continue to change over time, parenting can have important positive effects even when children have inherited a genetic vulnerability to problematic behaviors."

The paper is titled "Gene-Environment Contributions to the Development of Infant Vagal Reactivity: The Interaction of Dopamine and Maternal Sensitivity." The study was funded by the National Science Foundation.